Techelet-

There is a mitzvah in the Torah, to attach a string of Techelet on your Tsitsiot. About 1300 years ago the dye was lost, because of the romans. Now, there are two opinions to what the Techelet could be, one is the Murex Trunculous(Snail), and one is the Sepia Officinalis(Cuttlefish). The one I use is the Sepia Offincinalis. The reason why, is I looked in the Hilchot Ramabm, of Tzitzit, and this one matched the description a the Techelet more than the other. You may use either, or you may use just regular white Tsitsit. Those are the only two opinions that are reliable, so don't get any other ones. Any other, will make your Tsitsit not kosher. If you wear non-Kosher Tsitsit on Shabbat you are committing the sin of carrying(unless there is an Eruv).